1. Field of the Invention
The present invention relates to a GaN-based semiconductor light emitting diode, and more particularly to a GaN-based semiconductor light emitting diode in which a contact resistance at contact areas with electrodes is reduced and optical transmittance is improved, and a method for manufacturing the GaN-based semiconductor light emitting diode, thus obtaining a high luminance property at a constant driving voltage.
2. Description of the Related Art
Recently, LED displays, serving as visual information transmission media, starting from providing alpha-numerical data have been developed to provide various moving pictures such as CF images, graphics, video images, etc. Further, the LED displays have been developed so that light emitted from the displays is changed from a solid color into colors in a limited range using red and yellowish green LEDs and then into total natural colors using the red and yellowish green LEDs and a newly proposed GaN high-brightness blue LED. However, the yellowish green LED emits a beam having a brightness lower than those of the red and blue LEDs and a wavelength of 565 nm, which is unnecessary for displaying the three primary colors of light. Accordingly, with the yellowish green LED, it is impossible to substantially display the total natural colors. Thereafter, in order to solve the above problems, there has been produced a GaN high-brightness pure green LED, which emits a beam having a wavelength of 525 nm suitable for displaying the total natural colors.
Generally, the above-described GaN-based semiconductor light emitting diode is grown on an insulating sapphire substrate. Accordingly, differing from a GaAs-based semiconductor light emitting diode, an electrode is not formed on a rear surface of the substrate and both electrodes are all formed on a front surface of the substrate on which crystals are grown. FIG. 1 illustrates a structure of the above conventional GaN-based light emitting diode.
With reference to FIG. 1, a GaN-based light emitting diode 20 comprises a sapphire substrate 11, a lower clad layer 13 made of a first conductive semiconductor material, an active layer 14, and a second clad layer 15 made of a second conductive semiconductor material. Here, the first clad layer 13, the active layer 14 and the second clad layer 15 are sequentially formed on the sapphire substrate 11.
The lower clad layer 13 includes an n-type GaN layer 13a and an n-type AlGaN layer 13b. The active layer 14 includes an undoped InGaN layer having a multi-quantum well structure. The upper clad layer 15 includes a p-type GaN layer 15a and a p-type AlGaN layer 15b. Generally, semiconductor crystalline layers, i.e., the lower clad layer 13, the active layer 14 and the upper clad layer 15, are grown on the sapphire substrate 11 using a process such as the MOCVD (Metal Organic Chemical Vapor Deposition) method. In order to improve lattice matching of the n-type GaN layer 13a with the sapphire substrate 11, an AlN/GaN buffer layer (not shown) may be formed on the sapphire substrate 11 prior to the growth of the n-type GaN layer 13a thereon.
As described above, in order to form both electrodes on an upper surface of the electrically insulating sapphire substrate 11, designated portions of the upper clad layer 15 and the active layer 14 are removed by etching, thereby selectively exposing the lower clad layer 13, more specifically, the n-type GaN layer 13a, to the outside, and allowing a first electrode 21 to be formed on the exposed portion of the n-type GaN layer 13a. 
The p-type GaN layer 15a has a comparatively high resistance, and requires an additional layer for forming Ohmic contact serving as conventional electrodes. U.S. Pat. No. 5,563,422 (Applicant; Nichia Chemical Industries, Ltd., and Issue Date; Oct. 8, 1006) discloses a method for forming a transparent electrode 18 made of Ni/Au for forming Ohmic contact prior to the formation of a second electrode 22 on the p-type GaN layer 15a. The transparent electrode 18 increases a current injection area and forms Ohmic contact, thus reducing forward voltage (Vf).
Although the transparent electrode 18 made of Ni/Au is thermally treated, the transparent electrode 18 has a low transmittance of approximately 60% to 70%. The low transmittance of the transparent electrode 18 decreases overall light emitting efficiency of a package of the light emitting diode obtained by a wire-bonding method.
In order to solve the above low transmittance problem, there has been proposed an ITO (Indium Tin Oxide) layer having a transmittance of approximately 90% or more as a substitute for the Ni/Au layer. Since ITO has a weak adhesive force with GaN crystals and a work function of 4.7˜5.2 eV while the p-type GaN has a work function of 7.5 eV, in case that the ITO layer is directly deposited on the p-type GaN layer, Ohmic contact is not formed. Accordingly, in order to form Ohmic contact by reducing a difference of the work functions between the ITO layer and the p-type GaN layer, the conventional p-type GaN layer is doped with a material having a low work function such as Zn or is high-density doped with C, thus reducing the work function and allowing ITO to be deposited thereon. However, in case that Zn or C having a high mobility is used for a long period of time, Zn or C is diffused into the p-type GaN layer, thus deteriorating reliability of the obtained light emitting diode.
Accordingly, there have been required a GaN-based semiconductor light emitting diode, which forms Ohmic contact between a p-type GaN layer and electrodes and maintains a high transmittance in order to form the electrodes, and a method for manufacturing the GaN-based semiconductor light emitting diode.